Richard Lenski began passaging the bacteria in autumn of 1985. You can find experimental details online. His bacteria have now surpassed 50,000 generations during the past three decades. We shared a recent study (by R.E.Lenski) who has shown that fitness trajectories of Escherichia colipopulations, evolving over 50,000+ generations, were better described by a power-law model than by a hyperbolic model. According to the power-law model, the rate of fitness gain declines over time but fitness has no upper limit, whereas the hyperbolic model implies a finite limit.

Many populations live in environments subject to frequent biotic and abiotic changes. Nonetheless, it is interesting to ask whether an evolving population’s mean fitness can increase indefinitely, and potentially without any limit, even in a constant environment. Or whether it might push up against a finite ceiling and no longer evolve. It appears the former is the correct answer.

In attached publication, Lenski’s group examines whether the previously estimated power-law model predicts the fitness trajectory for an additional 10,000 generations. To that end, they conducted more than 1100 new competitive fitness assays. Consistent with the previous study, the power-law model fits the new data better than the hyperbolic model. They also analyzed the variability in fitness among populations, finding subtle, but significant, heterogeneity in mean fitness. Some, but not all, of this variation reflects differences in mutation rate that has evolved over time. Taken together, their results imply that both adaptation and divergence can continue indefinitely—or at least for a long time—even in a constant environment.